The use of stereotaxic radiosurgery to render tissue, and particularly tumorous tissue, necrotic is well known. While the radiosurgical principle of confining radiation as much as possible only to a particular volume is particularly attractive, problems regarding the precision of stereotaxic surgery remain an issue. With a risk that is proportional to both dose and the volume irradiated, radiation necrosis of tissue adjacent to a treated lesion remains the major complication of stereotaxic radiosurgery. Particularly, concerns remain as to whether particular volumes of tissue receive too much or too little radiation according to the prescription for treatment.
In stereotaxic radiosurgery, an accurate three-dimensional model of the skull or other tumor bearing portion of the body is generated from thin-cut CT scans, thus the volume requiring treatment can be visualized in three dimensions. A collimated radiation source is positioned in a sequence calculated to localize the energy deposition into a volume that as closely as possible conforms to that requiring treatment, while avoiding exposure of nearby healthy tissue. A system and method for performing stereotaxic surgery is disclosed in U.S. Pat. No. 5,207,223 issued to Adler on May 4, 1993 which is incorporated by reference herein. While this reference describes stereotaxic radiosurgery in which the treatment volume is accurately defined by moving the gamma beam in precisely defined arcs about the center of that volume, under that systems, the dose distribution is not defined or calibrated.
The dose distribution is an important parameter in stereotaxic surgery. If a radiation dose were too low due to unforeseen conditions at a point intended to receive the maximum radiation, then the surgery could be ineffective. If a radiation dose were too high at a particular point in the tissue, the surgery might have negative effects. Whether fixed or frameless stereotaxic radiosurgery is used, in order to determine if a particular scheme for the application of radiation beams to the tissue will result in a dose distribution within the prescribed limits and thereby optimize the treatment and minimize damage to healthy tissue, a system and method for such a determination is necessary. Thus, a system and method of calibrating and mapping the expected three-dimensional dose distribution that will be delivered by the collimated beam is desirable.